1. Field of the Invention
This invention relates to a light control apparatus and it particularly relates to a voltage control technology in which the voltage is applied to the ferroelectric substance.
2. Description of the Related Art
In recent years, a digital information recording system using the principle of hologram has been known as a large-capacity recording method (Patent Document 1, for instance).
The material that can be used for the spatial light modulator SLM of a hologram recording apparatus is, for instance, lead lanthanum zirconate titanate (hereinafter referred to as PLZT) or the like which has an electro-optical effect. PLZT is a transparent ceramic which has a composition of (Pb1 -yLay)(Zr1-xTix)O3. An electro-optical effect is a phenomenon in which an electric field applied to a substance causes polarization in the substance with a resulting change in its refractive index. Using the electro-optical effect, the phase of light can be switched by turning the applied voltage on and off. Accordingly, a light modulating material with an electro-optical effect can be applied to the optical shutter of a spatial light modulator SLM or the like.
In applications to these optical shutter and other devices, bulk PLZT has been widely used conventionally (Patent Document 2). However, it is difficult for the optical shutters using bulk PLZT to meet the demands for miniaturization or higher integration or the demands for lowered operating voltage or lowered cost. Besides, the bulk method involves a process of treating at high temperatures of 1000° C. or above after the mixing of material metal oxides, and hence, if applied to a device forming process, it may place many constraints on the selection of materials, the structure of the device and the like.
Under these circumstances, there have been attempts at applying a thin film of PLZT formed on a base material, instead of bulk PLZT, to light control devices. Described in Patent Document 3 is a display apparatus for which a PLZT film is formed on a glass or other transparent substrate and comb-shaped electrodes are placed thereon. This display apparatus has such a structure that a polarizing plate is provided on each of both faces of a display substrate with a PLZT film formed thereon. Here, the electrode terminal of each pixel is connected with an external drive circuit, so that desired pixels are driven and thus a desired display can be produced by the transmitted light from a light source provided on one face of the display substrate.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-297008.
[Patent Document 2]
Japanese Patent Application Laid-Open No. Hei05-257103.
[Patent Document 3]
Japanese Patent Application Laid-Open No. Hei07-146657.
The ferroelectric substance such as PLZT used in the light modulating material for the above-mentioned spatial light modulator is known to have an imprint phenomenon. This imprint phenomenon is a phenomenon where the amount of polarization produced in the ferroelectric substance, as a result of the electric field being continuously applied to the ferroelectric substance in the same direction, is held in memory. When a light control device is composed by use of the ferroelectric substance such as PLZT, continuously applying the electric field in the same direction causes an imprint phenomenon, so that the change in refractive index is shifted gradually. This shift of change in refractive index causes a problem of deteriorating the modulation accuracy of a spatial light modulator.